This invention relates to an electrical contact unit comprising at least one contact set for actuation by an electromagnet.
The invention has particular reference to an add-on block (having several contact sets arranged side-by-side) for detachable attachment to an electromagnetic relay itself including several contact sets also arranged side-by-side. The relay is arranged to apply an actuating force to all contact sets (its own and those of the add-on block) upon energization of an electromagnet included in the relay, the armature then moving against return springs also provided in the relay.
With normally closed contact sets of the prior art, energization of the electromagnet, with consequent attraction of the armature against the bias of the return springs, provides opening of each contact set. However, as the armature moves against the bias of the return springs, the respective biasing springs of the contact sets keep the contact sets closed for a substantial proportion of the armature movement. Thus, particularly if the electromagnet is energized slowly, the pressure between the mating contacts of each closed contact set decreases gradually until a balance is reached when there is no contact pressure left.